Chili peppers are served at the same temperature every other ingredient in a curry. Why, then, do we universally describe them as tasting hot? Mass synesthesia? Or something else?

There are plenty of instances in which people associate unlike things. The color red is almost always associated with heat, although the color is on the lower end of the energy spectrum of visible colors. The explanation for this is easy to come by: fire and heated metal generally have reddish tones. It's true that as the flames and metal get hotter, they shift their color to blue, but the red is what's rooted in everyday experience, and so it is associated with heat. Sight is a relatively concrete sense. Although people have different opinions on what they see, most of them see the same thing.

Taste is a much more slippery aspect of human existence. The very word, ‘taste,' has come to explain the inexplicably different way that people react to the same stimulus. They just have different tastes.

Many people, when given food laced with a new ingredient, are unable to identify what ingredient has been added to it, even if it is an ingredient with which they are familiar. Taste changes completely from person to person, depending on their personality, history, and recent experience (few want to follow a mouthful of scrambled eggs with grape jelly).

Despite all this variation, when people are given a jalapeno pepper, or a snoot full of cayenne; they'll reach for a glass of water, fan their tongues, and call it ‘hot'. When people are exposed to spice, they say that it ‘burns' them. They say it even if they don't taste is but accidentally touch a bit of it to their eyes, nose, or even their fingertips.
It can't burn them. It's not actually hot. Why do people say that it does?

Foods we call spicy or hot, contain capsaicin. There's nothing all too special about this particular chemical compound except for the fact that it gloms on to a receptor called VR1. That receptor sends a signal to the brain. VR1 is not primarily meant to let people know they're eating red hot chili peppers. Instead, it's meant to send signals to the brain when the body has been exposed to acids or to extreme heat. A pepper might not be hot in temperature, but once VR1 is triggered, that doesn't matter. The VR1 receptor can't send a signal to the brain saying ‘delicious pepper' any more than two cymbals, when clashed together, can make a sound like flute. Once the capsaicin binds to it, it rushes to let the brain know that something downstairs is burning, and it needs to send water.

Sadly, water won't soothe this burn. Casaicin is not water-soluble so it can stay intact in an ice bath. Sugar or fat will take it apart, so skip a glass of water and run for chocolate milk.